ddd

name	ddd
description	Domain-Driven Design (DDD) skill for the ikigai project

Domain-Driven Design (DDD)

Ubiquitous Language

Use these domain terms consistently in code, docs, and conversation.

Core Domain: Terminal-based AI Coding Agent

REPL - Read-Eval-Print Loop. The main interactive loop that reads user input, processes it, and displays results. The top-level orchestrator (ik_repl_ctx_t).

Scrollback - Immutable conversation history displayed above the separator. CRITICAL: Scrollback IS the context window - what you see is what the LLM sees (WYSIWYG). NOT just a display buffer.

Input Buffer - Editable multi-line user input area below the separator. Supports readline-style shortcuts and UTF-8.

Viewport - The visible portion of scrollback within terminal dimensions. Scrollable via Page Up/Down.

Layer - Independent rendering component in the layer cake architecture (scrollback layer, separator layer, input layer, spinner layer).

Session - A conversation instance. Created on startup or after /clear. Maps to database session record. Contains the active message array.

Message - Structured conversation unit with role (user/assistant/system/mark/rewind), content, timestamp, tokens, model. The fundamental unit of conversation history.

Context Window - The messages sent to the LLM. ALWAYS equals messages in scrollback. User controls this explicitly via /clear and message visibility.

Mark - Checkpoint in conversation history created by /mark command. Enables /rewind to restore context to earlier state.

Streaming - Progressive delivery of LLM response chunks. Displayed in real-time as they arrive via HTTP streaming.

Provider - LLM service (OpenAI, Anthropic, Google, X.AI). Abstracted behind unified interface using superset API approach.

Archive - Database storage of all messages. Permanent record, never auto-loaded into context. User searches and selectively loads.

Memory Management Domain

talloc Context - Hierarchical memory arena. Parent owns children. Free parent, free entire subtree. Root context owns everything.

Ownership - Clear parent-child relationships. Each allocation has exactly one owner responsible for cleanup.

Result Type - Return value carrying OK/ERR status with optional error context. Use CHECK() and TRY() macros for propagation.

PANIC - Unrecoverable error (OOM, data corruption). Immediately terminates process. NOT for recoverable errors.

Architectural Concepts

Three-Layer Separation:

1. Display Layer (scrollback) - Decorated, formatted, ephemeral. ANSI codes, colors, syntax highlighting applied at render time.
2. Active Context (session messages) - Structured ik_message_t[] array sent to LLM API.
3. Permanent Archive (database) - All messages ever, searchable, never auto-injected.

Scrollback-as-Context-Window Principle - The revolutionary insight: scrollback visibility = LLM context. User has explicit control. /clear = fresh context without data loss.

Single-threaded Event Loop - Sequential processing. No concurrency complexity. Terminal input → parse → mutate → render.

Direct Terminal Control - Raw mode ANSI escape sequences. No curses library. Single framebuffer write per frame.

Graceful Degradation - Never crash REPL for recoverable errors. Display errors inline. Continue accepting input.

Core Entities

ik_repl_ctx_t - Root aggregate. Owns all subsystems (terminal, scrollback, input, session messages, marks, LLM client, database).

ik_message_t - Value object. Immutable once created. Primary entity in conversation domain.

ik_scrollback_t - Entity managing display lines. Append-only buffer with O(1) arithmetic reflow.

ik_input_buffer_t - Entity managing user input. Mutable, supports multi-line editing.

ik_term_ctx_t - Entity for terminal state (raw mode, dimensions, capabilities).

Core Services

Rendering Service (render.c) - Builds framebuffer from layers, writes to terminal atomically.

LLM Client Service (openai.c, future: anthropic.c) - HTTP streaming to LLM providers. Abstracts API differences.

Database Service (db.c - future) - PostgreSQL persistence. Synchronous writes. Session and message management.

Config Service (config.c) - Loads ~/.config/ikigai/config.json. Provides API keys, model settings.

Command Service (commands.c) - Slash command dispatch (/clear, /mark, /rewind, /help, /model, /system).

Bounded Contexts

Terminal UI Context - REPL, scrollback, input, rendering, viewport, layers. Concerned with user interaction and display.

Conversation Context - Sessions, messages, marks, context window. Concerned with conversation structure and history.

LLM Integration Context - Providers, streaming, API requests, token management. Concerned with external AI services.

Persistence Context - Database, sessions table, messages table, full-text search. Concerned with permanent storage.

Memory Management Context - talloc contexts, ownership, Result types, error propagation. Cross-cutting concern.

Key Invariants

• Scrollback messages MUST match session_messages array (same content, same order)
• Session messages array MUST equal what gets sent to LLM
• Database writes MUST succeed before updating in-memory state
• Exactly one owner per allocation (talloc parent-child)
• 90% test coverage MUST be maintained
• Never run multiple make commands simultaneously

Anti-Patterns to Avoid

• Auto-loading messages from database into context (breaks explicit control)
• Hidden context injection (violates WYSIWYG principle)
• Using malloc/free instead of talloc (breaks hierarchical ownership)
• Recoverable errors that PANIC (PANIC is only for unrecoverable)
• Creating new files when editing existing ones works
• Over-engineering (KISS/YAGNI - build what's needed now)